1. Field of the Invention
The present invention relates to a lens tube unit to be employed in photographing equipment or optical equipment such as a camera, and a method for producing the same.
2. Related Background Art
A lens tube is composed plural tube members fitted together, and focusing or zooming operation is performed by helicoid screws, cams and cam followers, key members and key grooves, or spring members provided between such tube members for transmitting a rotary or translating force. Particularly a helicoid mechanism has an advantage of easily attaining a high precision in the lens movement, since a movement parallel to the optical axis is caused by mutual rotation of a female lens tube and a male lens tube.
On the other hand, in lens tubes, particularly in those for cameras, there are commonly employed so-called molded lens tubes, formed by plastic molding, for improving the precision of lens tube components and for reducing the production cost.
It is already commonly known to separately form the male and female tube members of a lens tube by plastic molding and to combine the thus formed tube members. Also the Japanese Patent Publication 4768/1985, corresponding to the U.S. Pat. No. 4,115,505 discloses a method of utilizing a female lens tube as a forming mold, and forming a male lens tube by injecting a resinous material into a molding cavity formed by said forming mold and the surfaces of plural mold members.
In case of separate plastic molding of the tube members constituting the lens tube, there is required an expensive molding apparatus in order to achieve a high finishing precision of the molds since otherwise the fitting of the female and male lens tubes has to be adjusted later, thus leading to an elevated cost of the lens tube.
On the other hand, the above-mentioned method disclosed in the Japanese Patent Publication 4768/1985 corresponding to the U.S. Pat. No. 4,115,505 can be effectively applied to a lens tube composed of a pair of male and female tube members, but is not suitable for a lens tube composed of a larger number of tube members. Also in the manufacture of a lens tube for use in a camera or the like, it is necessary to consider the precision of helicoids of the mutually fitted tube members, slidability of the fitted or screwed surfaces, and the realization of a desired strength in the tube members.
For example the Japanese Utility Model Application Laid-open 1009/1984, corresponding to the U.S. Pat. No. 4,309,076, discloses a lens tube unit composed of an intermediate tube provided with helicoids on the internal external peripheries and outer and inner tubes engaging with the helicoids of said intermediate tube. Such lens tube unit of three-layer structure composed of the outer, intermediate and inner tubes requires plural molding steps if it is manufactured by the method disclosed in the aforementioned Japanese Patent Publication 4768/1985.
On the other hand, the double helicoid structure as disclosed in the aforementioned Japanese Utility Model Application Laid-open 1009/1984 is associated with the problems of slidability of the screwed surfaces and the mechanical strength, in relation to the material constituting the lens tube members.
More specifically, polycarbonate resins are commonly employed for injection molding of lens tube members, and glass fibers are often mixed with such resins in order to improve the mechanical strength. Such glass fiber-reinforced polycarbonate resins are suitable for obtaining a sufficient strength in the lens tube, but if glass fibers are exposed on the fitted or screwed surface of the lens tube, it spoils the opposed surface. Also it is associated with the drawbacks of powder scattering and errors in optical precision after prolonged use.
Also in case of the aforementioned lens tube unit consisting of the outer, intermediate and inner tubes, these tube members perform rotary sliding motions relative to one another. Particularly in a lens tube for a camera, the smoothness of the rotating or sliding motion of the tube member has a significant influence on the feeling of the user in manual focusing operation.
The above-mentioned limitations in the slidability and in the mechanical strength cannot be resolved or reduced in the conventional method of manufacturing lens tubes.